The present invention relates to a multi-beam scanning device, and particularly to a synchronizing signal detecting method and device for the multi-beam scanning device.
In recent imaging apparatuses, such as a laser beam printer, there are ones employing a scanning device, which is provided with a plurality of laser diodes. In such a printer, by emitting a plurality of laser beam, which are modulated respectively, a plurality of line images are formed on a surface to be scanned, such as a photoconductive surface, simultaneously, thereby image forming speed becomes faster. Such a scanning device using a plurality of beams will be referred to as a multi-beam scanning device. In order to adjust the timing of drawing start position from which a scanning line is formed by each laser beam, a horizontal synchronizing signal should be detected for each laser beam.
In a conventional scanning device using a single laser beam, a beam detector as shown in FIG. 9A is employed.
FIG. 9A schematically shows a structure of the conventional scanning device using a single layer beam.
In FIG. 9A, a laser diode 1 emits a diverging laser beam, which is collimated by a collimating lens 2. The collimated laser beam is incident on a polygonal mirror 3, which is rotated at a high speed. The laser beam incident on reflection surface of the polygonal mirror 3 is deflected thereby to scan within a predetermined angular range. The deflected (i.e., scanning) beam passes through an fxcex8 lens 4 and is converged, in a direction perpendicular to the scanning direction, thereby so that a scanning beam spot is formed on a photoconductive drum 5. It should be noted that the photoconductive drum 5 is rotatable about an axis AX (see FIG. 9A) thereof, and the beam spot formed on the photoconductive drum moves, at a constant speed due to the function of the fxcex8 lens on the circumferential surface thereof in a direction parallel to the axis thereof. The direction in which the beam spot moves will be reflected to as an main scanning direction hereinafter. While the main scanning is performed, the photoconductive drum is rotated. The direction in which the circumferential surface moves with respect to the main scanning line (i.e., the direction perpendicular to the main scanning direction) will be referred to as an auxiliary scanning direction.
Within a scanning range of the laser beam, but outside a range contributing to the image formation, a beam detector 6 is provided. The beam detector 6 outputs a detection signal when the scanning beam is incident on the beam detector 6. The output of the beam detector 6 is used for generating a horizontal synchronizing signal.
FIG. 9B illustrates a light receiving surface of the beam detector 6, and a beam spot at various positions.
If the diameter of the beam spot in the main scanning direction on a plane including the light receiving surface of the beam detector 6 is smaller than the width of the light receiving surface of the beam detector 6 as shown in FIG. 9B, then when the laser beam traverses the light receiving surface of the beam detector 6, the output voltage VO varies as shown in FIG. 9C. By comparing the output voltage VO with a predetermined threshold value Vth, a horizontal synchronizing signal as shown in FIG. 9D is obtained.
If the beam detector as described above is used for a multi-beam scanning device, a plurality of beam spots traverse the light receiving surface of the beam detector 6 as shown in FIG. 10A. In FIG. 10A, two beam spots LB11 and LB12 scan on the beam detector 6. The beam detector 6 outputs the voltage VO which represents synthesized signals corresponding to the beam spots LB11 and LB12. Generally, in a multi-beam scanning device, the beam spots are aligned along a line that is inclined with respect to the main scanning direction.
If the distance between the two beams LB11 and LB12 is sufficiently larger than width, in the main scanning direction, of the beam detector 6, there is a period during which none of the two beams LB11 and LB12 are incident on the beam detector 6. In such a case, as shown in FIG. 10B, the beam detector 6 outputs a signal VO having two distinct peaks respectively corresponds to the two beams LB11 and LB12. Thus, the output signal VO shown in FIG. 10B can be used for generating the horizontal synchronizing signals for the laser beams LB11 and LB12, respectively, as shown in FIG. 10C. It should be noted, however, the two peaks of the output signal VO have similar waveforms, and therefore, it is impossible to determine which peak corresponds to which beam. In order to generate the horizontal synchronizing signals Hsync11 and Hsync12, a circuit for distinguish the two peaks should be added.
Further, if the scanning speed is relatively fast, i.e., the scanning period is shortened, and therefore the interval between the two beams, in the main scanning direction, relative to the scanning period becomes greater, and therefore, a period of time during which the beam detector 6 detects the laser beams LB11 and LB12 becomes shorter, a response of the beam detector 6 (i.e., a photo-electric conversion speed) may not follow the scanning speed. In such a case, the peaks of the signal VO cannot be identified. That is, if the waveform of the output signal of the beam detector 6 is likened to ridge and valley portions, the valley portion becomes shallower if the scanning speed becomes faster. Thus, also in such a case, a circuit for distinguish the peaks, which results in a large size of the scanning device.
If a distance between the two beams LB11 and LB12, in the main scanning direction, is small relative to the width of the beam detector 6, as shown in FIG. 11A, the output signal VO may have waveform as shown in FIG. 11B. The output signal VO includes components corresponding to the two beams LB11 and LB12, which are overlapped (see Hsync11 and Hsync12 shown in FIGS. 11C and 11D). It is impossible to detect the horizontal synchronizing signals for the two beams LB11 and LB12 from the signal VO alone. In order to distinguish the two beams, the output signal VO is compared with two threshold values Vth1 and Vth2 as shown in FIG. 11B. In FIG. 11B, positions A-E corresponds to the beam positions shown in FIG. 11A. At position B, the beam LB11 is detected to be incident on the beam detector but the beam LB12 has not yet been incident on the beam detector 6. At position C, the second beam LB12 is detected to be incident on the beam detector 6, beam LB11 being also incident on the beam detector 6. At position D, the beam LB11 has passed the beam detector 6, and at position E, the beam LB12 has passed the beam detector 6. Thus, by comparing the output signal VO with the threshold values Vth1 and Vth2, the components can be distinguished.
However, if the saturation level of the photodiode included in the beam detector is not set appropriately, the output VO of the beam detector 6 may have a waveform as shown in FIG. 11E. In such a case, it is difficult to distinguish the two components by comparing the output signal VO with the threshold values Vth1 and Vth2. Further, if the number of the beams is three or more, the same number of threshold values should be set, which make it more difficult to distinguish the components corresponding to the beams.
Alternatively, by providing a plurality of beam detectors respectively corresponding to the plurality of laser beams, the horizontal synchronizing signals can be detected accurately for respective beams. However, it is particularly very difficult to align a plurality of beam detectors at equivalent positions with respect to the corresponding beams. In particular, for three of more laser beams, such an alignment is practically impossible. Further, the beam detector is generally expensive, and thus, employing a plurality of beam detectors increases a manufacturing cost.
It is therefore an object of the invention to provide an improved method and device for detecting a synchronizing signal, which can be employed in a multi-beam scanning device. With the method and device, the synchronizing signal for each of a plurality of scanning beam can be detected accurately.
For the above object, according to the present invention, there is provided a method for detecting synchronizing signals of a plurality of scanning light beams in a multi-beam scanning device. The method includes modulating the plurality of light beams at predetermined different frequencies, respectively, receiving the plurality of light beams respectively modulated at different frequencies and outputting a signal representing intensity of received light beams, separating the signal representing the intensity of the received light beams into a plurality of component signals respectively corresponding to the plurality of different frequencies, and generating a plurality of synchronizing signals respectively corresponding to the plurality of light beams based on the plurality of component signals.
According to another aspect of the invention, there is provided a multi-beam scanning device, which is provided with a plurality of light sources that emit a plurality of light beams, respectively, a scanning system that deflects each of the plurality of light beams to scan within a predetermined scanning range, a modulating system that modulates the plurality of light beams at predetermined different frequencies, respectively, a beam detector positioned within the scanning range to receive the plurality of light beams respectively modulated at different frequencies, the beam detector outputting a signal representing intensity of received light beams, a separating system that separates the output signal of the beam detector, on a basis of the plurality of frequencies, into a plurality of component signals, and a synchronizing signal generating circuit that generates a plurality of synchronizing signals respectively corresponding to the plurality of light beams based on the plurality of component signals.
With this configuration, the laser beams emitted by a plurality of laser diodes are respectively modulated at different frequencies. The beam detector receives all the beams and outputs a signal representing a sum of all the components. The output of the beam detector is applied to a plurality of band pass filters corresponding to the frequencies at which the plurality of laser beams are modulated, respectively. Then, from the outputs of the band pass filters, a plurality of signals corresponding to respective frequencies are output. That is, each frequency components are separated. Once the beam detecting signals for respective laser beams are separated, the horizontal synchronizing signals can easily be obtained.
Preferably, the beam detector includes a single light receiving element capable of receiving the plurality of light beams, the beam detector outputting a sum of intensity of each of the plurality of light beams simultaneously incident on the beam detector.
In particular, the separating system may include a plurality of filters for separating the output signal output by the beam detector into the plurality of components respectively corresponding to the different frequencies, an amplifying system that adjusts a level of each of the components, and a plurality of one-shot multi vibrators respectively detects the components and generates a plurality of synchronizing signals respectively corresponds to the plurality of light beams.
Optionally, the plurality of light sources are composed of a plurality of laser diodes, respectively, and the modulating device includes an electrical current modulating device that modulates drive currents of the plurality of laser diode at the different frequencies, respectively.
Further optionally, the scanning system deflects the plurality of light beams so that beam spots formed by the plurality of light beams scan on a surface to be scanned in a main scanning direction, the beam detector is provided on optical paths of the plurality of light beams, the synchronizing signal generating system generates horizontal synchronizing signals for the plurality of light beams.
Still optionally, when an allowable jitter of a horizontal synchronizing of the plurality of light beams is defined by a following equation:
xcex94t=Te/n, xe2x80x83xe2x80x83
where Te is a one-dot period of the plurality of light beams scanning on a surface to be scanned, and n is an integer, then a minimum frequency fmin of the plurality of frequencies at which the modulating system modulates the plurality of light beams satisfy a following condition:
fminxe2x89xa6n/Te.